Probably read it in Modern Railways somewhere. But I notice someone has posted on another topic about a 110mph limit when hauling a HST as the Brake Control Unit isn't active.
Brake control unit will still be active, if the power car is shut down.
No speed restrictions on a single power car, and you can still get 125 downhill. You will not be allowed onto some sections of route though.
2 + 9 is the realistic limit for 125 running. If HSTs have a future, post regular service, I would see it as being in the charter market. Top speed not so important, so a longer set wouldn't be a problem, route permitting. If running a charter operation, you would want to maximise revenue, so maybe a different foramtion - more first class, possibly two catering vehicles. Packing the bodies in in Standard, isn't what it's about.
For preserved railways, minimum train lenght not an issue - that's for brake force at higher speeds. C3 width restrictions, storage space, facilities to maintain the things and likely customer interest, in what is always going to be a steam market, more of an issue.